One application for wide bandgap junction field effect transistors (JFETs) is in high voltage, high frequency power electronics. The exceptional device properties of wide bandgap JFETs make these devices capable of replacing high voltage insulated gate bipolar transistors (IGBTs) in a number of applications. Switching energy losses are one of the main characteristics of power semiconductor switches that are compared when selecting a device for new design. The transition speed is ultimately limited by the device. However, the performance of the gate driver can impact this speed considerably.
The main function of the gate driver is to deliver/remove the necessary gate charge required the by internal gate-source and Miller capacitance of the device in order for the device to transition between states. The faster the gate driver can perform this task, the faster the device will transition from off-state to the on-state, and from the on-state to the off-state. Therefore it is important to use a properly designed gate driver circuit for maximum performance of the device within a practical system application.
The gate structure of the JFET poses two distinct requirements in order to drive the device into conduction. These requirements are similar to a combination of a metal oxide semiconductor field effect transistor (MOSFET) and a bipolar junction transistor (BJT). First a high peak transient current is recommended for quickly charging the gate capacitance, like that of a MOSFET. Second a small DC gate current is required to maintain conduction, similar to a BJT.
An AC coupled, BJT-like RC driver can be used for wide bandgap JFETs in most applications. A driver of this type is depicted in FIG. 1. This driver solution has proven to provide exceptional switching performance, but experiences duty factor and switching frequency limitations. The RC driver is composed of a paralleled resistor and bypass capacitor connected between the gate/base of a semiconductor switch and the output of a Pulse Width Modulated (PWM) IC or other pulse generating circuit.
The RC driver is capable of level shifting, setting a DC current limit, as well as providing the high peak transient current required by most power semiconductors for a fast turn-on. To consistently maintain maximum switching speeds, the bypass capacitor of the RC driver must be full discharged prior to the next switching event. The time to discharge is depended on the RC time constant of the RC driver. Therefore the maximum switching frequency and duty factor of the application is limited by the RC time constant of the RC driver.
Accordingly, there still exists a need for improved gate drivers for wide bandgap JFETs and, in particular, an active, DC coupled driver that can overcome the limitations of the RC driver.